This invention relates to facsimile systems comprising a transmitter, a receiver and a communications network therebetween. More particularly, this invention relates to a system wherein a document is scanned in a facsimile transmitter to generate electrical information-bearing signals representing dark-light variations in the document being scannd. These information-bearing signals are transmitted over the communications network to a facsimile receiver where the information-bearing signals are converted to marks or images on a copy medium so as to form a copy which is a facsimile of the original document.
Typically the document at the transmitter circumscribes a rotatably mounted transmitter drum and the copy medium at the receiver similarly circumscribes a rotatably mounted receiver drum. The transmitter drum and the receiver drum are then rotated or revolved at substantially the same angular frequency so that the area of the document scanned by optical pickup means in a given period of time equals the area of the copy medium traversed by a writing mechanism such as a stylus in the receiver for the same period of time. In other words, the scanning rate of the transmitter as determined by the speed of rotation of the transmitter drum must be equal to, within very close tolerances, of the scanning rate at the receiver as determined by rotation of the receiver drum. In general, reliable facsimile transmission requires that the scanning rate of the transmitter and receiver be synchronized on a short term basis to an accuracy of one part in 1,000 during each revolution of the drum. On a long term basis (i.e., over the duration of the facsimile transmission), the scanning rates should be synchronized to one part in 20,000 and preferably to one part in 50,000 so as to avoid skew.
In order to achieve the necessary facsimile scanning rate control, facsimile transmitters, receivers or transceivers typically utilize synchronous motors which are driven by an AC source of known frequency. The AC source may comprise a power line where the equality of frequency at the transmitter and receiver is assured when the transmitter and receiver are operating on the same power grid. In the alternative, the reference frequency may be provided by a crystal oscillator as disclosed in copending application Ser. No. 493,119 filed July 30, 1974.
Although the use of synchronous motors with appropriate AC drives can achieve the necessary accuracy in scanning rates for facsimile operation, the synchronous motors do have certain drawbacks. First, an AC synchronous motor and associated drive mechanism is typically larger than a DC motor capable of driving the same load, and the size of the AC synchronous motor may, at times, present a packaging problem in a facsimile unit which should be relatively compact.
In addition, an AC synchronous motor typically utilizes more power than a DC motor and operates at higher voltages. Also, a DC is capable of operating over a wide range of scanning speeds so as to permit the operation of a facsimile unit in different scanning modes while an AC synchronous motor is limited to a relatively small range of speed near its designed synchronous speed. Finally, the less expensive AC motor may be acoustically more noisy than a DC motor and this can be a very important factor in an acoustically coupled facsimile transmission.
As set forth in the foregoing, the use of a DC motor in a facsimile unit is advantageous as compared with an AC synchronous motor in many respects. However, DC motors have not been utilized in facsimile units because of the difficulty in maintaining the necessary accuracy in scanning rates.